What does the author ‘+ing’ after ‘to’ in the following sentence?

Question

What does the author ‘+ing’ after ‘to’ in the following sentence?

These symbols, knows as conventional signs, are the ‘key’ to reading maps.

Answer 1

The noun "key" is often combined with the preposition "to". For example:

• The key to the front door. ("key" in the original, literal sense)
• Diet and exercise are the key to good health. (figurative meaning; something that is essential or provides access to something)
• Deep insights into your own study habits that are the key to finding more time for enjoyable language learning. (The Language Habit Toolkit)
• The key to the whole affair was (...) the revolver and the repeating rifles (...). (Quoted from Buffalo Bill in Bologna.)

There are also a number of expressions where "key" is combined with "for". For example,

• the key for turning of the radiator (i.e. the key's purpose is turning the radiator on or off)
• Raw wood and metal needs priming to provide a key for the topcoat. (How to buy the right paint for the job; this is about a degree of roughness that allows paint to stick to a surface.)

In the example cited in the question, "These symbols (...) are the 'key' to reading maps", the usage of "the key to" is similar to the second and third examples, which are about something that gives you access to something else. The "-ing" form ("reading") is known as a gerund, and "reading maps" is a noun phrase that can be combined with prepositions just like other noun phrases.

ELU SE has a few questions about "key to" versus "key for" that may also be helpful:

• “Key to” vs. “key for”;
• Using “to” versus “for” between two nouns (“key to success”).